[css-blocks] synthetic conflicts

There should be a way to create resolution constraints that span properties that don't naturally conflict in css. This can be used to implement things like accessibility constraints for contrast between foreground and background colors.

Such a capability should be extensible within an application or css library that is built on css-blocks.

The issue names no files, tests, or entry points. Start by surveying the constraint-resolution architecture and its extension points, then determine how cross-property constraints could be represented and implemented; done should include an extensible mechanism for synthetic conflicts such as foreground/background contrast.

Written by the indexing model from the issue text.